... it talks about using a 15K resistor, could i use a different resistor or will it be a big difference??

How 'different' will your new resistor be. How similar to his motor will yours be? I'm sure a 12K or 18K will be no better or worse a choice than his 15K. After all he did specify 10-20K above the picture. I'm pretty sure that he didn't do any extensive calculations before he came up with that range of values either, it's more likely that he happened to have a 15K resistor handy and it worked with his motor. I would be suspicious of the technical qualifications of someone who consistently refers to a potentiometer as a potmeter.